FEATHER ANALYSIS A technique for determining the range of a nuclide's beta rays in aluminum by comparing the absorption curve to that of a reference beta-emitting species. M.

FEATHER RULE A rule or equation for the maximum range of beta particles as a function of particle maximum energy. The equation is usually a linear one and when expressed in terms of mass per unit area does not vary widely with the atomic number of the absorber.

FISSION, NUCLEAR The spontaneous or induced division of a nucleus into two or more major parts, usually accompanied by the emission of neutrons, gamma radiation and, rarely, small charged nuclear fragments.

FISSION PRODUCTS Nuclides produced by fission and the daughter products of these nuclides.

FISSION, SPONTANEOUS Nuclear fission which occurs without the addition of particles or energy to the nucleus.

FISSION, THERMAL Fission caused by thermal neutrons.

FISSION YIELD The fraction of fissions giving rise to a particular nuclide before any nuclear decay has occurred.

FISSION YIELD, CUMULATIVE The fraction of fissions which have resulted in the production of a nuclide either directly or indirectly, up to a specified time. If no time is specified, the yield is considered to be the asymptotic value.

FISSION YIELD, DIRECT The fraction of fissions giving rise to a particular nuclide before any nuclear decay has occurred.

FISSION YIELD, INDEPENDENT The fraction of fissions which have resulted in the direct production of a given fission product nuclide.

FISSIONABLE See fissile.

FIXATION The incorporation of radioactive elements, usually fission products, into solid materials in such a way as to insure no significant release over long periods of exposure to the natural environment. NM.

FLOW COUNTER A radiation counter in which an appropriate atmosphere is maintained in a counter tube by allowing a suitable gas to flow through the volume. Such counters are operated in either the Geiger-Mller region or the proportional region. NM.

FLUENCE A measure of the time-integrated particle flux expressed in particles per unit area. M.

FLUOR A liquid or solid that is used in scintillation counters and that emits a flash of light when it is excited by radioactive or other radiation. B.

FLUORESCENCE Luminescence which occurs essentially only during the irradiation of a substance by electromagnetic radiation.

FLUORESCENCE YIELD For a given excited state of a specified atom , the ratio of the number of excited atoms which emit a photon to the total number of excited states.

FLUORIMETRY A method of analysis involving fluorescence by an analyte. M.

FLUOROGRAPHY Autoradiography involving the additional use of scintillation fluid or solid. Jones. Also known as photofluorography. NM.

FLUOROMETRY See fluorimetry.

FLUX The amount of a substance passing through an area per unit time. See also flux density.

FLUX DENSITY In a multicomponent mixture, the flux density NB of a species B is a vector which indicates the direction in which the species moves and the amount of substance of B passing through a plane perpendicular to the vector, divided by the time and by area. Sometimes the flux density is simply called flux.However, this is not consistent with the usage in vector theory. C.

FLUX DENSITY, ENERGY For mono-directional radiation , the energy traversing in a time interval a small area perpendicular to the direction of the energy flow, divided by that time interval and by that area.

FLUX DENSITY, NEUTRON See flux density, particle.

FLUX DENSITY, PARTICLE (OR, PHOTON) At a given point in space, the number of particles or photons incident in a time interval on a suitably small sphere centered at that point, divided by the cross-sectional area of that sphere and by that time interval. The particle flux density is identical with the product of the particle density and the average speed of the particles.

FLUX DENSITY, 2200-METRE-PER-SECOND A fictitious flux density expressed as the product of the total number of neutrons per cubic metre and a neutron speed of 2200 metres per second.

FLUX DEPRESSION The lowering of the particle flux density in the neighborhood of a sample due to absorption of particles in the sample.

FLUX MONITOR A known amount of activatable material irradiated together with a sample; the induced radioactivity is used as a measure for a particular flux density during the irradiation.

FLUX PERTURBATION The lowering of the flux density in a sample as a result of both flux depression and self shielding.Sometimes a specified flux density can increase as a result of the change of the energy of the particles in the sample, e.g., the increase of the thermal neutron flux density in hydrogen containing material.

FNAA Fast neutron activation analysis.

FOIL DETECTOR A small piece of foil used to measure flux densities by activation.

FORWARD ALPHA SCATTERING TECHNIQUE Determination of elastically scattered alpha particles with a detector at forward angles; the method uses the energy of the detected particle to deduce the scattering mass for light target elements.

FOUR- COUNTING Measuring radiation from a source with a detector or detector system that covers all directions from the source; that is, with a 100% geometry factor.

FRACTION, BOUND In radioassays , the fraction of the incubation mixture which, after separation, contains the analyte bound to the binding reagent.

FRACTION, FREE In radioassays , the fraction of the incubation mixture which, after separation, does not contain the bound analyte.

FRACTIONAL CRYSTALLIZATION A class of chemical separation by fractionation which takes advantage of slight differences in solubility of different compounds in a solvent, the least soluble crystallizing first and removed from the mother liquor.

FRENKEL DEFECT A crystal defect consisting of a vacancy and an interstitial atom which arises when an atom is moved out of a normal lattice site and forced into an interstitial position. M.

FREQUENCY OF EMISSION In imaging with gamma rays, it is the branching fraction of each gamma ray. NM.

FRICKE DOSIMETER A dosimeter based on the oxidation of aqueous Fe+2 to Fe+3 by ionizing radiation. Ch.

ft VALUE The product of the half-life of a beta-decaying nuclide and a function (the Fermi function) that depends on the decay energy and transition type. M.

FUEL CYCLE The sequence of steps, such as mining, processing, fabrication, utilization, reprocessing, and refabrication, through which nuclear fuel may pass.

FUEL ELEMENT The smallest structurally discrete part of a reactor which has fuel as its principal constituent.

FUEL, NUCLEAR Material containing fissile nuclides , which when placed in a reactor enables a chain reaction to be achieved.

FUEL REPROCESSING The processing of nuclear fuel , after its use in a reactor, to remove fission products and recover fissile and fertile material.

FULL ENERGY PEAK In a radiation spectrum , the part of the spectral response curve corresponding to the total energy of the detected radiation.

FULL ENERGY PEAK EFFICIENCY, ABSOLUTE For a radiation spectrometer viewing a source of radiation, the fraction of events of a given energy radiation that are registered in that radiation's full energy peak.

FULL ENERGY PEAK EFFICIENCY, INTRINSIC The detector efficiency when considering only events where the total energy of the radiation is absorbed in the sensitive volume of the detector.

FULL WIDTH AT HALF MAXIMUM (FWHM) In a distribution curve comprising a single peak, the width of that peak at half its maximum amplitude.

FULL WIDTH AT TENTH MAXIMUM (FWTM) In a distribution curve comprising a single peak, the width of that peak at one-tenth its maximum amplitude.